Stuck in the 70th
Plenty of Apps existing in mobile stores (as well on Desktop-Stores) rely on a payment model as old and as broken as always, the end user licence fee: With a one-time fee you buy yourself access into the product and are allowed to use it. This model has been around since the seventieth but is as broken it was back than as it is now. And with even the big player like Microsoft and Adobe moving away from it into a service-based model, it has clearly been proven bad.
Considering that aside from a games and some utilities, most apps are unable to work without their online counter part, being merely a pretty way to access an online service given to, the one-time fee App-Store enforce on the distributors is completely broken. Because it means you have to pay the running costs of your service solely by the money provided from new customers. Making your focus shift away from delivering a great user experience for your existing customers to market your app to new customers.
Making exactly that mistake, 37Signals are warning you about in their book Rework, when they talk about “be at-home good”: the focus of the product should always lie in the best service to your existing customers. And in order to ensure that, you need a monetization model, which serves that, like subscription fees. Only then could you continue offering your service. Just consider how much money Spotify would have to ask for if they wanted to offer their service as a one-time-fee to their customers. You get my point …